This invention relates to a multi-layer structure permanent magnet and a method for manufacturing the same using an extrusion process to form the multi-layer structure.
Permanent magnets are used in many applications. One such application is in the creation of brushless electric motors where they are used to replace the traditional alnico or ceramic magnets. Such magnets are useful, but are limited in terms of energy versus size. Higher energy in a smaller package is generally desirable in most industrial and commercial settings. Rare earth magnets, because of their extremely high energy, are finding increasing use in applications such as brushless DC motors.
The manufacture of rare earth magnets is a laborious and expensive process. As is described in U.S. Pat. No. 4,902,357, the production of rare earth magnets begins with sintering rare earth material. Following the sintering process, the magnet is solution treated and aged at elevated temperatures to achieve the desired magnetic properties.
Rare earth magnets are also very difficult to machine because they are mechanically hard and brittle. It is simply not practical to machine rare earth magnets or structures containing such magnets after they are magnetized during the manufacturing process steps, because the machined particles adhere strongly to the magnets. Because of the extremely high coercive forces of rare earth magnets it is also not practical to construct a magnetized rotor or stator of rare earth magnets and then remagnetize the structure as is commonly done with alnico magnets.
Because commercial desire is strong to employ rare earth magnets in applications such as DC brushless motors, the industry continues to look for methods of manufacturing rare earth magnets that are low in cost and are produced to near net shapes, thus having virtually no machining or relatively reduced machining.
The present invention is directed to an extrusion method for producing a multi-layered structure with high-energy ring magnets at low cost and which can be readily machined to its final size. More specifically, the invention is directed to a method for producing a multilayer rod having the desired magnetic properties and ease or limited need of machining.
In this invention an extrusion process is employed. A first layer of low alloy steel powder is injected into a chamber of an extrusion machine. A rare earth metal powder is then injected into the extrusion chamber to form a circumferentially disposed second layer around the first layer. The material so disposed in the extrusion chamber is then extruded through a die to form a rod with the concentric layering intact and having magnetic properties not found in the base materials
A third layer, if desired, may be extruded around the rare earth metal at the same time. The third layer if used forms an outer skin of the extruded rod and protects the rare earth metal layer from mechanical fragmentation and corrosion. If a third layer is not desired an antioxidant coating may be applied. Then a stainless steel or an aluminum retention cap is inserted to provide protection against magnet integrity.